Adjusting HF Receiversbrarc.org/wp-content/uploads/Adjusting_HF_Receivers.pdf · “If you can’t...
Transcript of Adjusting HF Receiversbrarc.org/wp-content/uploads/Adjusting_HF_Receivers.pdf · “If you can’t...
ADJUSTING YOUR
HF RECEIVER
N5KIP
January 31, 2017
Disclaimers
• What works on one model of radio might
not work well on another
• CW (narrow bandwidth) and SSB (wider
bandwidth) will require different receiver
adjustments
• Optimal receiver settings will change as the
noise level changes
It’s the Antenna Dummy!
• Your antenna is far more important than the
number of receiver controls on your rig or
what price category of transceiver you have
• There are great receivers in QRP kit radios that
have only RF gain, AF gain and perhaps a filter
control
• Don’t be concerned if your vintage or entry level
transceiver doesn’t have all the features
discussed
“If you can’t hear ‘em,
you can’t work ‘em”
• Think of conversation in a noisy room
• To carry on a conversation, you must hear the
person trying to talk to you:
• He or she must talk louder (increase signal
strength) and/or
• Lower the noise by moving to a corner, cup your
hands to your ear etc.
(reduce receiver noise)
Squelch Control
• Very useful on FM VHF and UHF
• Steps in squelch adjustment for SSB and
CW HF operating:
• 1 Locate the squelch control
• 2 Place squelch control in open position
• 3 Apply super glue (okay, I’m kidding)
AF and RF Gain Adjustments
• Beginners are prone to turn up the RF gain
all the way (more gain must be better,
right?) and use the AF gain as a
conventional volume control
• Try reducing RF gain by turning up AF gain
and using the RF as the “volume control”
resulting in a lower noise floor
Methods of RF Gain Adjustment
• Here are a few methods which reduce RF gain to
result in a better signal to noise ratio.
• All seem to work
• Again, simply think in terms of turning up the AF
gain and using the RF gain as a volume control
(Bob Heil (K9EID) has a You Tube video showing how on SSB he
reduces RF gain in noisy conditions. He also turns off pre-amp, may
add attenuation and uses slow ACG)
Quick & Easy RF Gain Adjustment
• Adjust RF Gain to the “sweet spot”
• Turn RF and AF gain all the way down
• Turn off ACG (I cheat and leave the ACG on)
• Protect hearing, avoid headphones when ACG off
• Turn up AF gain about midway then slowly turn up RF gain listening for signal clarity
• Turn on ACG again if desired or if using headphones
Another Easy RF Gain Adjustment
• Set AGC to fast
• Tune to an unoccupied frequency, check the S-meter
reading of the noise and if the meter is moving at all,
reduce the RF Gain until the meter just stops moving.
• On many radios reducing the RF gain will cause the S-
meter reading to rise because the S meter is actually
reading from the RF gain circuit rather than actual signal
strength. That's fine because what we are looking for is
the point where the S-meter movement stops.
• For example, if you have S4 noise, reduce RF gain such
that the S meter only deflects on greater than S4 signals
More Precise Adjustment
• 1 Set audio noise floor
• 2 Set receiver noise floor
• 3 Connect antenna
• 4 Determine if preamp is needed
• 5 Determine if attenuator is needed
• Turn off ACG, preamp and attenuator and disconnect antenna
• Set AF gain and RF gain to minimum
• Advance AF gain until just hear noise (audio noise floor)
• Turn on preamp
• Advance RF gain until you hear receiver noise just above the
audio noise floor
• Connect antenna and tune unoccupied frequency. You should
hear band noise
• Switch off pre-amp. If still hear band noise, pre-amp not
needed and proceed to next step. If no longer hear band
noise, turn on preamp and adjustment is complete
• If preamp not required, add ATT in steps. When no longer hear
band noise, decrease ATT one step and adjustment is
complete. (ATT may only have one step depending upon rig)
Preamp and Attenuator
• Preamp is typically needed only on higher
frequencies with little noise. I virtually never use
the preamp on HF
• Attenuators are most often used to reduce
overload from very strong signals
• Attenuators may actually also improve S/N ratio
on bands with lots of noise
• I often use ATT on 80, 40, 30 and even 20 meters
due to high noise at my location
• Similar in concept to reducing RF gain
Example
S5 noise
unoccupied frequency
< S1 noise (ATT on)
unoccupied frequency
Nearby 40 meter SSB signal Nearby 40 meter SSB signal
S/N ratio (in S units) about 9 to 5
50.2/3.2 microvolts = 16.7
S/N ratio (in S units) about 6 to 1
6.3/0.2 microvolts = 31.5
Attenuator Off Attenuator On
Illustration
•CW signal on 20 meters
•Note drop in S meter with ATT on
•Despite drop in S meter, the signal
is easier to copy with the
attenuator on due to decreased
noise level
Automatic Gain Control (AGC)
• A closed-loop feedback regulating circuit which
provides a controlled output signal amplitude, in
spite of variation of the amplitude of the input
signal.
• ACG limits variation in volume between signals of
differing strength
• Ham radio receivers typically allow for ACG
adjustment as slow, fast or off.
ACG Adjustment
• Fast ACG may work against you with fluttering
signals (polar path for example)
• Try slow ACG on SSB. Audio may “pump” with
fast ACG
• Fast ACG may help with weak CW signals
• Consider turning off ACG for weak signals if RF
gain is properly adjusted
IF Filters for SSB
• SSB filters are typically in the range of 1.8 to 3.0 kHz
• Narrower the filter, the less the noise but at expense of
audio fidelity.
• I have set the SSB filters on my IC 7200 to 1.9, 2.4 and
3.0 kHz and use the 2.4 kHz filter about 90% of the time
• On the IC 7200, the filter shape can be set “soft” (gentle
roll off) or “sharp” (steep roll off).
• Many favor the sharp filter shape but I have not
developed a preference. Your mileage may vary
IF Filters for CW
• CW filters are typically in the range of 100 to 600 Hz
• I set my IC 7200 DSP CW filters at 600, 400 and 250 Hz
• In my experience, a narrow filter is quite helpful in
reducing noise and bringing out a weak CW signal
• May hear “ringing” with very narrow filters (less so with
DSP compared to mechanical filters)
• I tune the band listening for “CQ” using a wide filter, zero
beat, then narrow the filter for a QSO
• If calling CQ, a wide filter allows you to hear slightly off
frequency responders
Filter “Slope”
• Your rig may allow adjustment of filter slope
(“shape”)
• Sharp slope (rapid roll off) is said to be preferable
• To my ears, a filter with a sharp slope is virtually
indistinguishable from a somewhat narrower filter
with a soft slope
• You will develop your own preferences when it
comes to filter widths and slopes
Getting a Sense of Filters
Soft Slope
Sharp Slope
Sharp Slope
40 Meter SSB Noise with 1 kHz DSP Filter
Icom IC 7200 Sharp and Soft Slope Alternated
Use your digital software (in this case, Digital Master 780)
to get an idea of what different filters “look like”
Bandpass Filters
• Used to adjust the low and high frequency cutoff
• Dual bandpass filters function as variable filter (widens or
narrows the passband like a regular filter)
• I have occasionally found it useful to cut the lower audio
frequencies which contain considerable noise (rumble)
but little essential audio from the normal human voice
• On my IC 7200, I rotate either of the twin passband
controls clockwise to cut these lower frequencies
• Sometimes cutting the higher frequencies seems to help
Let’s “Play The Price is Right”
Passive Audio Bandpass Filter
What’s the Correct Price?
A $ 3.64
B $ 23.99
C $ 79.99
D $ 129.99
Let’s Play “The Price is Right”
Passive Audio Bandpass Filter
A $ 3.64 Correct Answer
B $ 23.99
C $ 79.99
D $ 129.99
Simple Acoustic Cavity CW Filter
• PVC elbow from the hardware store
• Mine resonates best at about 800 Hz
• Others have built plastic or wood chambers, used
plastic tumblers etc. Size and shape of the cavity
determines the resonant frequency
• See Wikipedia for formulas applicable to different
shapes (closed tube, open tube, rectangular,
sphere etc.)
• A larger cavity resonates at a lower frequency
Elbow Alone Resonates
About 850 Hz
Elbow and Extension Resonates
About 600 Hz
Picking Up Good Vibrations
February 2017
QST
In this case,
a speaker is built
into the base of the PVC elbow
Notch Filters
• Automatic notch filters eliminate a narrow constant
frequency tone (or multiple tones) from a SSB signal.
• Audio quality may be degraded, especially if the
automatic notch filter tries to eliminate multiple tones
• Manual notch filters eliminate a single narrow frequency
tone which is selected by rotating the control knob. It is
considered by many superior to the automatic notch filter
• The width of the manual notch filter may be adjustable
• I may be lazy, but the automatic notch filter worked great
on the few occasions I’ve needed it. Never have needed
the manual filter
NB and DSP NR
• Noise blankers (NB) work well to control short bursts of
energy like auto ignition and light dimmers
• Digital Signal Processing (DSP) Noise Reduction (NR) is
often helpful in dealing with noise
• Step 1 A/D conversion
• Step 2 Signal processing in the form of computer performed
complex mathematical operations on the digital signal (7 -10 dB
noise reduction is typical per ARRL testing)
• Step 3 D/A conversion
• Depending upon your rig, there may be more than one
NR protocol available and you may be able to adjust the
amount of processing desired
RIT and XIT
• Split operation is common among rare DX stations but
also found in the QRP Fox Hunts and similar activities
where there may be pileups. Listen for “up” or “down”
• Work split using RIT, XIT or dual VFO’s
• Receiver Incremental Tuning (RIT or “clarifier”) allows you
to set the frequency on which you wish to transmit then
listen on a nearby frequency
• Transmitter Incremental Tuning (XIT) allows you to listen
on the transmit frequency of a station working split and
automatically transmit up or down in frequency where the
DX station is likely listening.
Importance of Ambient Noise
• Human hearing has about 100 dB range
• Library like room has noise level of about 40 dB
leaving only 60 dB to play with
• Encapsulating headphones reduce noise by
about 15-25 dB
• Quality in ear monitors reduce noise about 25 dB
• In ear monitor and external ear muffs reduce
noise about 40 dB
• Once you regularly use headphones, you’re likely
to list that speaker on ebay
A Word About Menus
Reset Is Your Friend
• My entry level IC 7200 has 32 menu items not including
the menus associated with front panel controls
• Even if each menu had only two possible options (most
have multiple), that’s over 1000 permutations
• Stuff happens
• Keep a record of the menu selections and control settings
that you have found to be useful
• In the event of malfunction, reset!
Video References
• https://www.youtube.com/watch?v=W0tnt8MXYwE
(You Tube Amateur Extra lesson 6.4 on Filters and
Impedance Matching)
• https://www.youtube.com/watch?v=1sX0VsINGu8f
(Bob Heil,K9EID, on ACG and RF gain adjustment from
Ham Nation episode 130)
• https://www.youtube.com/watch?v=hM_ID-tivsA Radio
Terminology explained AGC Automatic Gain Control
• https://www.youtube.com/watch?v=9_uYHwAQcQ0
Setting "split" on a Kenwood TS-590S
• https://www.youtube.com/watch?v=26oCCEZRANk
Icom IC-7200 Notch Filter Demo
References
• John (K5JS) kindly provided a great online reference.
Look for “Noisy K3…” at http://www.w3fpr.com/
• “For The New General Class Operator: HF Radio SSB Phone Receiver
Functions” see:
http://www.barrowhamradio.org/images/articles/New%20HF%20Operator%20
-%20HF%20Radio%20SSB%20Phone%20Receiver%20Functions.pdf